1. Field of the Invention
The present invention relates to systems and methods that are used to separate molecular hydrogen from a volume of gas. More particularly, the present invention is related to systems and methods that separate hydrogen from a volume of mixed gas by exposing the mixed gas to a hydrogen permeable material through which only atomic hydrogen can readily pass.
2. Background Art
In industry, there are many applications for the use of molecular hydrogen. However, in many common processes that produce hydrogen, the hydrogen gas produced is not pure. Rather, when hydrogen is produced, the resultant gas is often contaminated with water vapor, hydrocarbons and/or other contaminants. In many instances, however, it is desired to have ultra pure hydrogen. In the art, ultra pure hydrogen is commonly considered to be hydrogen having purity levels of at least 99.999%. In order to achieve such purity levels, hydrogen gas must be actively separated from its contaminants.
In the prior art, one of the most common ways to purify contaminated hydrogen gas is to pass the gas through a conduit made of a hydrogen permeable material, such as palladium or a palladium alloy. As the contaminated hydrogen gas passes through the conduit, atomic hydrogen permeates through the walls of the conduit, thereby separating from the contaminants. In such prior art processes, the conduit is kept internally pressurized and is typically heated to at least three hundred degrees centigrade. Within the conduit, molecular hydrogen disassociates into atomic hydrogen on the surface of the conduit and the conduit absorbs the atomic hydrogen. The atomic hydrogen permeates through the conduit from a high pressure side of the conduit to a low pressure side of the conduit. Once at the low pressure side of the conduit, the atomic hydrogen recombines to form molecular hydrogen. The molecular hydrogen that passes through the walls of the conduit can then be collected for use. Such prior art systems are exemplified by U.S. Pat. No. 5,614,001 to Kosaka et al., entitled Hydrogen Separator, Hydrogen Separating Apparatus And Method For Manufacturing Hydrogen Separator.
The flow rate of hydrogen gas through the walls of a conduit is proportional to the length of the conduit and the thickness of the walls of the conduit. Thus, a highly efficient purification system would have a very long, very thin conduit to maximize flow rate. However, palladium is a very expensive precious metal. Consequently, conduits made of palladium and palladium alloys are very expensive to manufacture. As such, it is desirable to use as little of the palladium as possible in manufacturing a hydrogen gas purification system. Furthermore, conduits made from palladium and palladium alloys typically hold gas under pressure and at high temperatures. Accordingly, the walls of the conduit cannot be made too thin, otherwise the conduit will either rupture or collapse depending upon the pressure gradient present across the wall of the conduit.
A typical prior art hydrogen separator uses coils made from palladium tubing. Coils are used to maximize the surface area of the palladium within a confined space. However, there are many disadvantages associated with the use of coiled tubing. In order for a palladium based hydrogen separator to work, it must be heated to a temperature in excess of 300 degrees Centigrade. As palladium coils are heated to such temperatures, they expand. Furthermore, as hydrogen diffuses through the walls of the palladium coils, the palladium expands significantly. As a palladium coil is repeatedly expanded and contracted, the palladium coil twists. The twisting of the palladium coils fatigues the palladium and causes the palladium to become brittle. Eventually, a palladium coil will crack and the hydrogen separator will cease to work.
Another disadvantage of hydrogen separators that use coiled palladium tubing is that the coils of palladium are very susceptible to vibration damage. The palladium coils within a hydrogen separator act as springs. If the hydrogen separator experiences any vibrations during operation, those vibrations resonate within the palladium coils, causing the palladium coils to move. As the palladium coils resonate and move, the palladium experiences fatigue and becomes brittle. This eventually causes the palladium coils to crack and fail.
Yet another disadvantage of hydrogen separators that use palladium coils is that of contaminant gas back-up. If a hydrocarbon rich gas is introduced into a palladium coil, some hydrogen will disassociate from the hydrocarbon and will pass through the wall of the palladium coil. What is left behind in the palladium coil is mostly carbon and oxygen, which forms carbon dioxide and carbon monoxide. The carbon dioxide and carbon monoxide fill the palladium tube. New hydrocarbon gas must therefore diffuse through this contaminant gas before it can reach the surface of the palladium coil. If there is a large back-up of contaminant gas, the hydrocarbons may take a very long time to reach a palladium surface. Hydrogen in the supply gas must be able to reach the palladium surface in a time frame that is short compared to the residence time of gas in the coil. However, the concentration of the non hydrogen component in the supply gas will increase gradually as more and more hydrogen is removed as the gas stream progresses through the coil. This greatly reduces the efficiency of the hydrogen separator. If the flow in the palladium gas is increased to flush out contaminant gas, hydrocarbons may flow through the palladium tubing before ever having a chance to lose hydrogen through the palladium. This too greatly reduces the efficiency of the hydrogen separator.
To further complicate matters, conduits made from palladium and palladium alloys may become less efficient over time as the interior walls of the conduits become clogged with contaminants. In order to elongate the life of such conduits, many manufacturers attempt to clean the conduits by reverse pressurizing the conduits. In such a procedure, the exterior of the conduit is exposed to pressurized hydrogen. The hydrogen passes through the conduit wall and into the interior of the conduit. As the hydrogen passes into the interior of the conduit, the hydrogen may remove some of the contaminants that have deposited on the interior wall of the conduit.
Due to the generally cylindrical shape of most prior art hydrogen purification conduits, the conduits are capable of withstanding a fairly high pressure gradient when the interior of the conduit is pressurized higher than the exterior of the conduit. However, when such conduits are cleaned and the external pressure of the conduit is raised higher than the interior pressure, a much lower pressure gradient must be used, otherwise the conduit will implode.
In the prior art, improved conduit designs have been developed that attempt to minimize the amount of palladium used in a conduit, yet increase the strength of the conduit. One such prior art device is shown in U.S. Pat. No. 4,699,637 to Iniotakis, entitled Hydrogen Permeation Membrane. In the Iniotakis patent, a thin layer of palladium is reinforced between two layers of mesh. The laminar structure is then rolled into a conduit. Such a structure uses less palladium. However, the conduit is incapable of holding the same pressure gradient as solid palladium conduits. Accordingly, the increase in efficiency provided by the thinner palladium layer is partially offset by the decreased pressure limits, and thus gas flow rate, that are capable of being processed.
A need therefore exists for a hydrogen separator that uses palladium tubing, yet greatly reduces the susceptibility of the palladium tubing to cycle fatigue damage.
A need also exists for a hydrogen separator that optimizes the exposure of gas to palladium surfaces, thereby minimizing the need for hydrogen to diffuse through contaminant gases.
Lastly, a need exists for a hydrogen separator that enables a large concentration of hydrocarbon gas to pass through palladium conduits without having the hydrocarbons being swept out of the conduits by the flow of gas.
These needs are met by the present invention as described and claimed below.